The main objective of the proposed studies is to address some fundamental, but as yet unresolved, problems in radiation biology by taking advantage of flow cytometric tools available at Los Alamos National Laboratory. The precision of cell survival measurements at low doses can be significantly improved by using cell sorting based on electronic volume signals . In addition, this method can be used to measure radiation response as a function of position in the cell cycle using asynchronous cell populations for many types of cultured cells commonly used by radiation biologists, thus avoiding the problems associated with cell synchronization techniques. This volume cell sorting method will be applied to the study of age response and oxygen effect using cultured cells and multicellular spheroids. Specifically, we proposed to address the following questions: 1) Is the oxygen effect purely a dose modifying factor, or does its magnitude change with radiation dose? 2) Does the oxygen effect vary with position in the cell cycle? 3) Do cells with a short G1phase have only one peak of radioresistance (in late Sphase), unlike cells with a long G1phase (which are also resistant in early G1-phase)? 4) Does the reduction of radiosensitivity as a function of cell cycle position for highLET radiations depend on the cell type? 5) What is the relative importance of the oxygen effect versus the cell cycle redistribution which the lack of oxygen induces in chronically hypoxic cells? This investigation uses the recognized potential of flow cytometry as a practical tool to improve understanding of some of the basic problems in radiation biology.